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1. Tian Gui (reproductive essence generated congenitally) originates from the innate vital Qi of life, while Jing Xue (essence and blood) is transformed from postnatal food essence to form the body. At age sixteen (two times eight), Tian Gui arrives in men; belonging to Yang and corresponding to the Sun, their semen becomes increasingly abundant. At age fourteen (two times seven), Tian Gui arrives in women; belonging to Yin and corresponding to the Moon, their menstruation flows regularly. When man and woman join their essences, offspring are conceived, following the principle that the Heavenly Path forms the male and the Earthly Path forms the female.

[Note] Tian Gui is the innate true Qi bestowed by parents, which generates the body's vital energy. Jing Xue is transformed from water and grain (dietary nutrients) and serves as the foundation for forming the physical body in postnatal life. At age sixteen, a man's innate Kidney Qi is vigorous, Tian Gui arrives, and merges with the postnatal-generated essence, becoming abundant. Since men belong to Yang, and Yang corresponds to the Sun, when essence is abundant, erections occur daily. At age fourteen, a woman's innate Kidney Qi is solid, Tian Gui arrives, and merges with the postnatal-generated blood, becoming abundant. Since women belong to Yin, and Yin corresponds to the Moon, when blood is abundant, menstruation occurs monthly. Therefore, when men and women unite at the appropriate age, their innate true Qi and postnatal essence and blood harmonize and merge through Yin and Yang, resulting in conception. During this time of Yin-Yang union, if Yang predominates, a male fetus naturally forms, which is the Heavenly Path forming the male; if Yin predominates, a female fetus naturally forms, which is the Earthly Path forming the female.

2. Although men experience seminal emission at sixteen, they must wait until thirty to marry; although women begin menstruation at fourteen, they must wait until twenty to wed. This is all to ensure that Yin and Yang are fully mature and substantial, so that the children born will be robust, strong, and enjoy extended longevity.

[Note] Men experience seminal emission at sixteen but must wait until thirty to marry; women begin menstruation at fourteen but must wait until twenty to wed. This is done to desire that Yin and Yang become complete and solid. Only then can intercourse lead to pregnancy, pregnancy to gestation, and the child raised will inevitably be sturdy, strong, and long-lived. Nowadays, young women who have not yet undergone the capping ceremony, whose Tian Gui has just begun, engage in sexual activity too early. This causes Yin Qi to leak prematurely, damaging the body before it is fully developed and agitating the Yuan Qi before it is substantial. Consequently, even if intercourse occurs, conception may fail, or gestation may not be sustained, and any child born will inevitably be fragile and short-lived.

3. For men, preserving essence lies in reducing desires. Intercourse must be timed correctly and not missed. One must wait for the arrival of the Yinyun (ovulatory fertile window); the intense joy and urge to nurture offspring at this moment constitute the true mechanism of conception.

[Note] The method for preserving essence lies solely in minimizing desires. When engaging in intercourse with a woman, one must seize the correct timing and not miss the window due to being too early or too late. Generally, after a woman's menstrual flow ends once a month, there is a specific day of Yinyun. During this time, internal heat rises like steam, the state is akin to drunkenness or infatuation, and there is an unbearable urge for intercourse. This is a natural physiological rhythm and constitutes the true mechanism for conception and biological transformation.